Scientists use light echoes to re-observe erupting star

NASA images show the development of the Eta Carinae light echo over the past decade

An international team of scientists, including two astronomers from McMaster University, has been able to re-observe an event first seen on Earth more than 150 years ago.

Eta Carinae, a double star, is one of the most luminous and energetic stars in the entire Milky Way galaxy. It grew substantially brighter between 1837 and 1858 during the Great Eruption, an unstable period in its life cycle comparable to a cosmic hiccup.

The disturbances that caused the brightening, believed to be precursors of the star's death, had actually occurred roughly 7,500 years earlier, with the light taking that long to reach Earth, where 19th century observers lacked the ability to record what they were seeing.

New research findings, published today in the journal Nature, tell the story of what happened during that very rare eruption, after modern-day scientists observed and measured light echoes of the same events.

"We can do things now that they wouldn't have dreamt of in the 19th century," says Doug Welch, a professor of physics and astronomy at McMaster and one of the authors of the paper. "It's still, to me, extremely cool that we can see something again, for the first time."

Light echoes are created when original light reflects off an object and changes direction.

When the bright light from Eta Carinae emanated from the star in all directions, observers on Earth first noticed the changes via direct observation.

The same light also bounced off space dust and traveled to Earth as an echo. During the 150-year detour, science and technology advanced enough to make it possible to measure the events based on the reflected light, effectively allowing scientists of today to reach back in time.

What they observed showed that the outburst was in fact much cooler than they had anticipated, causing them to rethink the status of Eta Carinae in relation to others in its class, known as Luminous Blue Variables.

This isn't the first time scientists have used light echoes, says Welch, but it is the first time they have used them to study this event.

The team of 16 scientists that conducted the research worked under the leadership of Armin Rest of the Space Telescope Science Institute in Baltimore, Maryland, and included scientists from as far away as Chile and England. Such international collaboration is common in the field of astronomy, especially with the advent of the Internet, Welch explains.

The team used sophisticated analysis of the colour, quality and strength of the light to measure Eta Carinaes disturbances. Researching such events as the Great Eruption allows scientists to understand how massive stars influence galaxies and the formation of other stars, Welch says.

The results provide information about how the universe is changing over time, including clues to the how many generations of stars went into making the material that composes Earth and our own bodies, he says. That in turn helps with questions of how easily life could form or not form in other places in the universe.

At McMaster, Welch was involved in modeling the event and writing large sections of the research paper for the team, while PhD student Brendan Sinnott calculated how to estimate the parameters of the space dust and the duration of the event.

"It's really amazing that we're able see this eruption that was observed over 150 years ago on Earth in the light echoes, and then re-observe that same event with multimillion dollar astronomical equipment," says Sinnott. "The fact that this re-observation came with an interesting result, that the eruption was much cooler than we expected, is really what makes science fun."